Plants and factories utilize process control devices to control the flow of fluids in processes, wherein “fluids” include liquids, gasses, or any mixture that can flow through a pipe. Manufacturing processes that create consumer articles or goods such as fuel, food, and clothes require control valves to control and regulate fluid flow. Even a medium sized factory may utilize hundreds of control valves to control a process. Control valves have been utilized for over a century, during which time valve designers have continuously improved the operational performance of control valves.
When designing a process, the designer is faced with many design requirements and design constraints. While a valve must be able to provide the required flow capacity when the valve is in the full open position, minimizing the body size of a valve for a particular application provides many benefits. Minimizing the valve size reduces the cost of the valve itself and reduces the cost of the actuator that controls the valve. More importantly, a valve with a smaller body provides improved performance and stability because a smaller valve has less gain, more accuracy and better flexibility than a larger valve. In addition to creating process instabilities and other control system uncertainties, oversized valves require more frequent plug movement to accurately control flowing fluids, causing increased wear on the valve components and the valve actuator. Oversized valves also increase process variability. Process variability is very serious because it results in an inefficiency which is an expense that continues twenty-four hours a day, seven days a week (or whenever the process is running). Thus, a lower cost, smaller valve can provide superior performance if it can meet the maximum flow requirements.
Some process control applications require a valve to maximize flow in two directions, often called a “bi-directional flow application.” In a typical bi-directional valve, there is a vertical segment where fluid flows upward. Therefore, flow in one direction is often referred to as flow-up and flow in the other direction is referred to as flow-down. In a majority of applications, valve selection is predicated on the direction of flow through the valve. Skirted globe valves are popular for bi-directional flow applications because they can be utilized regardless of flow direction. A skirted plug is supported by the skirt as the plug slides up and down, guided by an annular valve seat in the valve body irrespective of flow direction. In some cases the annular valve seat serves a dual role as a bearing surface for the plug skirt and as a sealing surface for mating with the sealing surface on the plug. In particular, the skirt acts as a guide to stabilize the valve plug within the valve as fluidic forces place a side load on the plug. The skirted plug allows the valve stem to be reduced in size because the valve plug is supported from both ends (i.e. the stem and the skirt).
A smaller valve stem provides multiple benefits including minimization of the force required to move the plug because there is less friction on the stem from the valve stem packing and seal. Smaller valve stems are also easier to seal because there is less force on the seal due to reduced surface area. Minimizing the size of the valve stem also minimizes the size of the actuator required to move the valve plug due to reduced operating friction. Reduced friction also provides improved plug reaction time and better overall valve performance.
One inherent problem with utilizing a skirted globe valve is that the valve plug is never fully removed from the valve seat. As a result, skirt material obstructs the flow path and reduces the amount of flow in a full open condition. In addition to decreasing maximum capacity by decreasing the diameter of the flow path, the skirt obstruction produces vortices, turbulence and pressure gradients in the flowing fluid which results in hydrodynamic drag. Thus, the obstruction presented by the skirt prevents the valve from producing the maximum flow properties found in other valve types having the same port size.